The present invention relates to a method of manufacturing pieces made from thermostable composite materials from fibrous materials in which synthetic resin powders are incorporated, said fibrous materials thus obtained having great flexibility, being then subjected to a heat forming element in a mold for obtaining rigid pieces made from composite materials indeformable to heat at a temperature of at least 150.degree. C.
It was already known how to incorporate plastic material powder in a fibrous material in the roving condition, for example by passing it through a fluidized bed of powder. It was noted that the use of such a method operating on compact rovings did not always allow the incorporation of a sufficient proportion of powder.
The preimpregnated materials of the prior art are formed from fibrous elements such as mats, boards, sheets in which resins are introduced in the form of powder by sintering or electrostatic powdering. These resins, in order to be held in these elements, are melted and not cross linked, otherwise these elements could no be handled. Such melting results in connection of the fibers and these become rigid or semirigid.
Other cases of reacting elements have also been contemplated, such as stacks of film generating products or powder stacked with fabrics or mats. In both cases, the reaction obtained during formation of the composite material only gives a roving coating, that is to say the viscosity of these products in the molten state is too high to penetrate inside the rovings and the unitary fiber.
The work carried out for increasing the proportion of powders incorporated in the fibrous material has shown an important relationship to be complied with between the grain size of the powder and the diameter of the unitary filaments forming the fibrous material.
Finally, the work on which the present invention is based has allowed the nature of the materials forming the synthetic resin powder and that of the synthetic resin forming the coating to be determined with great accuracy so as to form pairs of both resins in which the specificity of the bonding between the partners allows surprising results to be obtained of capital importance in the field of use, namely the possibility of obtaining composite materials resisting severe temperature conditions in which, contrary to the prior process, are obtained by treatment at temperatures allowing equipment of current quality to be used, that is to say working at extrusion and forming temperatures not exceeding 180.degree. C., whereas before, to reach comparable results, it was indispensable to operate at very high temperatures, for example in the order of 260.degree. C., which is the melting temperature of a 6.6 polyamide.
In addition, the melting viscosity of the plastic powders described in the prior art is such that it is difficult to completely eliminate bubbles when passing through heat in a mold, even by exerting high pressure.
The choice of modified heat hardenable or thermoplastic powders having by nature much lower melting viscosities allows the problem to be overcome and forms one of the essential features of the invention.